1. Field of the Invention
This invention relates to acismic methods and systems having improved signal-to-noise ratios and having the capacity for discriminating closely adjacent geological formations or discontinuities and small anomalies, and is particularly applicable to the systematic surveying or exploring of extended geographical areas.
2. Description of the Prior Art
In the well-known reflection method of making seismic surveys, a seismic impulse such as an explosive shot is initiated, and a record is made of impulses received at sensors or detectors at spaced locations along a seismic cable extending from the shot-point. The seismic sensors usually used on land are known as geophones, and those usually used in marine seismic cables are termed hydrophones. The seismic waves from the seismic impulse are reflected back to the surface from interfaces between geologic layers of different properties or characteristics, to the sensors located over the area under survey. The reflected signals received at the sensors are transmitted to recording and processing equipment in a ship or seismic vehicle.
In the course of undertaking a survey of this type for an extended geographical area or prospect, the prospect is covered by a grid of survey lines, and seismic profiles are recorded along these survey lines. In marine work a seismic streamer cable is continuously towed through the water along one of the survey lines, and seismic impulses are initiated from the ship at regular intervals such as every 10 or 20 seconds. On land, the seismic cable is characteristically in the form of a spread of a series of identical sections laid on the ground and connected together by plug-type electrical connectors. The line of survey through the prospect is traversed by firing a seismic shot with the recording and data processing equipment located in a recording truck, connected into the cable. After the shot has been fired and the seismic data have been recorded, one or more of the cable sections are disconnected from one end of the spread, moved, and reconnected to the other end of the spread along the direction of travel. A multiple switch in the recording truck is advanced to a new position thereby advancing the portion of the cable connected to the data processing equipment one or more cable-section lengths along the survey line, whereupon a new recording cycle is undertaken.
Characteristically, in accordance with the prior art, in a 10,000-foot marine streamer cable, approximately 1,500 sensors have been employed. Groups of about 30 of these sensors have been electrically connected, so that seismic signals from typically 48 seismic channels are transmitted from the cable. These reflected signals are recorded and displayed in parallel traces to visually portray subsurface features of the geologic area under survey. In the absence of adjustment, the reflected signals appearing in traces, originating with hydrophone groups remote from the shot-point would be displaced with respect to those in traces originating near the shot-point, and would introduce an apparent variation in depth in the representation of a horizontal reflecting interface. In accordance with known techniques, a moveout or angularity correction is applied to adjust the adjacent traces on the display, so as to present a faithful representation of the reflecting interfaces. The value of the moveout correction is a function of time from the shot, the average seismic-wave velocity in the earth, and the distance between shot point and the detector groups.
In such reflection-type seismic systems which have been developed up to the present time, as noted above, it is conventional to electrically interconnect about 30 geophones or hydrophones into a single group which has a physical extent along the cable from about 100 to 300 feet; one commonly used distance is about 230 feet. With this arrangement, as noted above, at least 48 conductor pairs are employed to transmit the signals from the 48 groups of sensors to the recorder, normally located in the ship or truck used for transporting the seismic equipment.
Usually the frequency response of such known systems is in the very low frequency end of the spectrum, from about 5 to 40 hertz, with the peak response lying below 20 hertz. Among other factors, the lowered high-frequency response is attributable to phase differences of signals arriving at spaced apart points along the length of the groups of electrically-connected sensors which normally extend for a distance of about 230 feet, as mentioned above. To avoid cancellation of signals which are arriving at the various sensors, the length of the electrically connected sensor units or groups preferably should be relatively small as compared with the wavelength of the seismic signals which are being received.
To extend the analysis on a more quantitative basis, a group of electrically-connected seismic sensors disposed on the earth's surface will be considered, the group having a length s. If a seismic wave traveling horizontally along the array is incident at one end of the array, the time T required for the wave to traverse the array is EQU T=s/v (A)
where v is the acoustic velocity of the propagating medium adjacent to the array. In water, the acoustic velocity is approximately 5,000 feet per second, hence the transit time T of a horizontally traveling wave in water for a 230-foot group of electrically interconnected sensors would be 0.046 second. For additive reinforcement along the length of the group or unit of sensors, the group length should be less than about one-quarter wavelength. The time for such a seismic wave to travel one wavelength is 0.046.times.4, or about 0.184 second, which is the period of a wave having a frequency of about 6 hertz, or six cycles per second. Waves traveling along the array and having frequencies substantially greater than the 6-hertz cutoff limit tend to be cancelled. When the length of the group is exactly one-half wavelength, the response of the group is zero since the wave is totally cancelled.
Of course, as is well known, seismic waves may be incident upon a sensor group from many angles. For example, seismic waves reflected from deep geologic formations propagate towards the sensor group in a near-vertical direction. The wave fronts are detected nearly simultaneously by all the detectors in one group; accordingly, in the absence of near surface irregularities such as weathering or elevation differences, the upper cutoff frequency is virtually infinite.
On the other hand, the travel path of seismic waves reflected from very shallow earth layers, whose depth is significantly less than the distance from the shot to the sensor group, approaches the horizontal, so that the foregoing analysis for horizontally traveling waves is applicable. Consider, for example, the relatively flat incident angle of a seismic wave reflected from a layer 1000 feet deep with respect to a sensor group 10,000 feet away from the shot point.
Typically seismic reflections from shallow layers are relatively rich in the high frequencies (100-500 hertz) that are useful for high-resolution analysis of details of geological features. Unfortunately, presently-employed seismic systems, with sensor group lengths of from 100 to 300 feet, are selectively responsive to very low frequencies. with the frequencies below 20 cycles per second predominating; and desired high-frequency waves from shallow earth layers will be cancelled by us of the long sensor groups.
Of course, the emphasis on the lower frequencies in conventional large-area prospecting systems, limits the sensitivity and the power of the system to detect and resolve closely-spaced geological layers, minor discontinuities, or other significant features which may not be extensive in size, particularly in the shallow part of the geological section. Further, now that many of the prospects of principal interest have been surveyed on a reconnaissance basis, it is becoming increasingly important to employ geophysical surveying techniques of high resolution for detail work.
Detector groups of considerable length have been used preferentially in reflection seismic exploration in order to discriminate between signals and unwanted noise. The general theory explaining the relation of array length to the signal-to-noise ratio may be found in the paper The Moveout Filter by Savit, Brustad, and Sider, "Geophysics", January 1958.
From time to time, attempts have been made to improve the high-frequency response of seismic arrays by using very short arrays. Unfortunately, the most common result was a considerable degradation of data quality owing to the inevitable decrease in signal-to-noise ratio.
Some exploration seismologists gave preliminary consideration to the possiblity that the final signal-to-noise ratio could be restored to a value comparable to that of arrays in common use by greatly increasing the number of arrays, in effect retaining the number of individual detectors in common use but subdividing them into many more, but shorter, arrays. However, two principal factors combined to render this procedure impractical. The first had to do with processing procedures. In order to determine the normal moveout corrections to be used in assembling data, correlation (or equivalent) processes have to be used among sets of individual seismic traces (data from individual arrays). With data from short arrays, the poor signal-to-noise ratios reduced the effectiveness of this process. Furthermore, the vastly increased number of individual data recordings increased data reduction (processing) costs to levels unacceptable for commercial prospects. Moreover, many other problems and complexities made the implementing of such a system appear to be an insurmountable project.
More specifically, another difficulty resulting from the use of shorter groups of sensors is that, for example, if a full length two-mile seismic cable is to be employed, and if the sensor density is to remain unchanged, the number of signal channels which must be connected to the recorder is increased by an order of magnitude as the group length is reduced. This would mean that about ten times as many conductor pairs would have to be added, if the group lengths were to be significantly reduced. This, of course, would greatly increase the number of contacts required in connector plugs used to couple cable sections made according to previously-proposed techniques. Further, the large number of conductors would greatly increase the weight and the bulk of the cables and decrease their flexibility to unacceptable levels.
Typical prior art systems which are useful in reviewing the background of the present invention include U.S. Pat. No. 3,133,262 of Booth B. Strange, "Dual Seismic Surveying System", granted May 12, 1964, which discloses two overlapping hydrophone spreads in a single marine cable; U.S. Pat. No. Re. 25,204 of Carl H. Becker, "Method of Geophysical Exploration"; C. H. Savit, et al., U.S. Pat. No. 3,096,846, granted July 9, 1963, entitled "Method and Apparatus for Seismographic Exploration", which discloses fixed tapering and weighting of seismic signals from different seismic sensors to provide directivity; J. P. Woods, et al., U.S. Pat. No. 3,346,068, which discloses directionally sensitive seismic transmitting and receiving arrangements; C. E. Welles, U.S. Pat. No. 3,689,873, which discloses some delay and weighting circuits for seismic signals; and, R. G. Quay, U.S. Pat. No. 3,613,071, which shows the use of two arrays or geophones, having different spacing and different sampling rates.